It's like if private sector unions got to choose the board and executives at their company. While that would doubtless be good for union employees (or at least leaders), it would be quite deleterious to the interests of shareholders.
Public employee union dues are especially odious since the government is effectively collecting a payroll tax to bribe itself by way of a colluding intermediary. This is morally even worse than politicians being bribed by some private sector party.
Fun aside: given the empirical proof that only money speaks already given, the statement "If you deprive labor of a voice then only money speaks" is always true, regardless of whether or not labor has a voice.
[1]https://www.cambridge.org/core/journals/perspectives-on-poli...